Boreal forests play a major role in climate change because of their expansive land area and ability to store large quantities of carbon.  Quantifying and explaining tree water use along a soil drainage gradient improves predictive understanding of the influence of physiological processes on carbon, water, and energy cycles.  Four stands burned in 1850, 1930, 1964, and 1989 were chosen because previous studies determined these stands to represent critical stages of forest development that capture both species composition and leaf area changes.  In all stands, sap flux of 111 Picea mariana, Pinus banksiana, and Populus tremuloides trees, leaf water potential (Ψ_L) of 95 trees, and various environmental covariates were measured along a soil drainage gradient to determine patterns and factors influencing spatial variability of tree transpiration.  We hypothesized that 1) tree transpiration will differ between drainage conditions due to tree size and age and drainage-related hydraulic adjustment and 2) tree size can be explained by species-specific growth differences with changing soil conditions. 

Results/Conclusions

Average mid-day Picea mariana Ψ_L for all four burns showed no significant difference between well- and poorly-drained sites (-1.23 & -1.29 MPa respectively) (p-value = 0.576).  Ψ_L growing season averages for Picea mariana resulted in significantly different values of -0.29, -0.44, -0.42, -0.34 for pre-dawn and -1.09, -1.37, -1.20, -1.25 MPa for mid-day at the 1989, 1964, 1930, and 1850 burns respectively.  Water use results of Picea mariana using 204 additional sap flux sensors at the 1964 stand differed between drainage conditions when expressed per unit xylem area with trees in poorly-drained soils experiencing higher rates than trees in well-drained areas (101.79 & 83.02 g cm^-2 day^-1 respectively).  In contrast, when expressed as transpiration per tree, taking tree size into account, trees on well-drained soils had higher rates than those in poorly-drained locations (366.96 & 216.82 g tree^-1 day^-1 respectively).  The presence of Pinus banksiana and Populus tremuloides increased stand transpiration rates for the well-drained areas considerably as compared to poorly-drained areas.  This indicates that tree size, which is constrained by growth and anaerobic conditions across well- to poorly-drained areas, is driving differences in tree transpiration along this drainage gradient.  Results also show that stand age has more influence on Picea mariana Ψ_L and, likely, stand transpiration per unit leaf area than soil drainage. This suggests that canopy conductance is influenced by soil drainage because transpiration varies between well- and poorly-drained areas while Ψ_L does not.